BackgroundBioturbators affect multiple biogeochemical interactions and have been suggested as suitable candidates to mitigate organic matter loading in marine sediments. However, predicting the effects of bioturbators at an ecosystem level can be difficult due to their complex positive and negative interactions with the microbial community.Methodology/Principal FindingsWe quantified the effects of deposit-feeding sea cucumbers on benthic algal biomass (microphytobenthos, MPB), bacterial abundance, and the sediment–seawater exchange of dissolved oxygen and nutrients. The sea cucumbers increased the efflux of inorganic nitrogen (ammonium, NH4 +) from organically enriched sediments, which stimulated algal productivity. Grazing by the sea cucumbers on MPB (evidenced by pheopigments), however, caused a net negative effect on primary producer biomass and total oxygen production. Further, there was an increased abundance of bacteria in sediment with sea cucumbers, suggesting facilitation. The sea cucumbers increased the ratio of oxygen consumption to production in surface sediment by shifting the microbial balance from producers to decomposers. This shift explains the increased efflux of inorganic nitrogen and concordant reduction in organic matter content in sediment with bioturbators.Conclusions/SignificanceOur study demonstrates the functional role and potential of sea cucumbers to ameliorate some of the adverse effects of organic matter enrichment in coastal ecosystems.

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Culture of the mussel Mytilus galloprovincialis in a South African bay created organic enrichment and anoxia in sediments. Particulate organic matter (POM) was high under rafts versus the references, especially in the first 10 cm (C = 7.5 versus 0.4%, N = 0.7 versus 0.08%). Total reducible sulphides (TRS) increased threefold downcore (from 0.04 to 0.12%). High C:N ratios (12–15) indicated accumulation of refractory POM, derived mainly from faeces and decaying mussels and foulers. Although O2 uptake by raft sediments was the lowest, rates could not conclusively be separated from the references. Ammonium dominated N efflux, the highest and most variable rates being under mussels (825 ± 500 µmol NH4·m–2·h–1). Phosphate efflux (25–140 µmol·m–2·h–1) could not be ascribed to culture biodeposition, but there was an inconclusive trend for the molar N:P ratio to be highest in these sediments. Macrofauna biomass was reduced and trophic groups and taxa altered. Under rafts, macrofauna and organic debris were linked to O2 uptake rates, whereas at the reference sites, macrofauna appeared to be the major O2 consumer. It was concluded that POM and TRS in sediment as well as macrofauna biomass, and potentially molar N:P ratios, were more sensitive indicators of benthic impact from mussel culture than O2 uptake rates or nutrient fluxes.

The sea cucumber Apostichopus japonicus is an important aquaculture species in China. As global interest in sustainable aquaculture grows, the species has increasingly been used for co-culture in integrated multitrophic aquaculture (IMTA). To provide a basis for optimising stocking density in IMTA systems, we parameterised and validated a standard dynamic energy budget (DEB) model for the sea cucumber. The covariation method was used to estimate parameters of the model with the DEBtool package. The method is based on minimisation of the weighted sum of squared deviation for datasets and model predictions in one single-step procedure. Implementation of the package requires meaningful initial values of parameters, which were estimated using non-linear regression. Parameterisation of the model suggested that the accuracy of the lower (T L ) and upper (T H ) boundaries of tolerance temperatures are particularly important, as these would trigger the unique behaviour of the sea cucumber for hibernation and aestivation. After parameterisation, the model was validated with datasets from a shellfish aquaculture environment in which sea cucumbers were co-cultured with the scallop Chlamys farreri and Pacific oyster Crassostrea gigas at various combinations of density. The model was also applied to a land-based pond culture environment where the sea cucumber underwent a fast growth period in spring and non-growth periods during winter hibernation and summer aestivation. Application of the model to datasets showed that the model is capable of simulating the physiological behaviour of the sea cucumber and responds adequately to the wide range of environmental and culture conditions.